The subject matter disclosed herein relates to the art of turbomachines and, more particularly, to a dual fuel combustor nozzle for a turbomachine.
The regulatory requirements for low emissions from gas turbine power plants have grown more stringent over the years. Environmental agencies throughout the world are now requiring even lower rates of emissions of NOx and other pollutants from both new and existing gas turbines. Traditional methods of reducing NOx emissions from combustion turbines (water and steam injection) are limited in their ability to reach the extremely low levels required in many localities.
Dry Low NOx (DLN) systems integrate a staged premixed combustion process, gas turbine controls, fuel, and associated systems. Such systems may include two principal measures of performance. The first measure of performance is meeting emission levels required at baseload on both gas and oil fuel, and controlling variations of those levels across the load range of the gas turbine. The second measure of performance is system operability. Design of a DLN combustion system also requires hardware features and operational methods that simultaneously allow an equivalence ratio and a residence time in the flame zone (combustion parameters critical to emission control) to be low enough to achieve low NOx, but with acceptable levels of combustion noise (dynamics), stability at part load operation, and sufficient time for CO burnout.
DLN combustors are in wide use. While effective, DLN combustors were designed mainly for natural gas combustion. New customer demands may require the combustors to have wider fuel flexibility in view of availability of alternative gas fuels and increased cost for natural gas fuel. More specifically, customers may require a combustor capable of running with a blended synthesis gas (syngas) and also capable of running with natural gas alone (dual fuel flexible). Syngas is a mixture of hydrogen and carbon monoxide and sometimes carbon dioxide. Blended syngas may be a mixture of natural gas/hydrogen/carbon monoxide. Syngas is combustible and is often used as a fuel source but has less than half the volumetric energy density of natural gas. As a volumetric flow rate for syngas must be more than double the volumetric flow rate of natural gas for the same combustion flame temperature, syngas fuel pressure ratio will be extremely high (over 1.7) if the same primary nozzle presently used for natural gas fuel is also used for operation with syngas. Such a high fuel pressure ratios may increase system hardware and operational costs.
Existing dual fuel nozzles direct one fuel, through a central nozzle portion and another fuel through an outer conduit portion that extends about the central nozzle portion. Both fuels then emerge from an outlet portion of the nozzle into a combustion chamber, mix, and are ignited. When only one fuel is being utilized, an air purge is required to prevent a back flow of hot combustion products or reactant gases from the combustor into one of the central nozzle portion and outer conduit portion. Typically, when using only one fuel, that fuel is passed though the outer conduit portion and air is passed through the central nozzle portion. The air purge requires additional components and plumbing for the combustor. More specifically, a compressor is required to supply the air for the purge and additional piping and valves are required to switch between the second fuel and the air purge.